The subject invention relates generally to image printing devices, and in particular, multi-color image printing devices.
Existing printing devices employ a plurality of laser beams to print multi-color images. To this end, a multi-color image printing device scans a plurality of laser beams across at least one photosensitive member in a scanning direction. The laser beams are modulated, i.e., turned ON and OFF, according to image data or xe2x80x9cprint dataxe2x80x9d in order to create latent images in the form of lines on the photosensitive member(s). A common clock signal controls the frequency at which the laser beams are modulated as the laser beams scan across the photosensitive member(s) at a constant speed. Each laser beam creates a latent image on a photosensitive member corresponding to a respective color, such as cyan, magenta, yellow or black.
Each laser beam is emitted and scanned by a respective laser scanning unit (LSU), which includes a laser printhead for emitting the laser beam and a scanning unit for scanning the laser beam across a photosensitive surface. Each laser beam passes through at least one lens before impinging upon the photosensitive surface. The lenses through which the laser beams pass have slightly different magnification levels due to manufacturing variations. Moreover, even if two laser beams pass through a same lens, they enter the lens at different points and they exit the lens at different points, resulting in different levels of magnification for the two laser beams.
Each laser beam is reflected off of a rotating polygon mirror in order to scan the laser beam across an imaging member, which can be a photosensitive surface. Each LSU is capable of locking onto a common rotational speed. Each scanning unit has one or more sensors to achieve the rotational speed lock. A start-of-scan sensor and/or an end-of-scan sensor at the edges of the photosensitive surface sense the laser beam as it scans thereacross. The sensor(s) produce a horizontal synchronization signal (HSYNC) which is used to synchronize the start of scanning for each of the laser beams. Thus, all of the laser beams begin their scanning across the photosensitive surface(s) at a same point in the scanning direction.
However, due to the differences in magnification described above, the laser beams conclude their scanning across the photosensitive surface(s) at different points in the scanning direction. That is, the raster lines created by the laser beams have different lengths. This misregistration of the differently colored raster lines causes the differently colored toners to be superimposed upon each other incorrectly, resulting in an inaccurate and visually unappealing toned image on the print medium.
Accordingly, there is a need for a multi-color image printing arrangement that prints raster lines of different colors that have the same length and that can be registered with each other in the horizontal, scanning direction.
The above needs, as well as others, are fulfilled by providing a method and arrangement for adjusting the lengths of raster lines produced by a plurality of printheads. The adjustment can be performed for each laser printhead independently such that each printhead produces registered raster lines of an equal length. A separate clock signal is provided for each printhead to control the frequency at which each printhead is modulated according to print data. Adjustments of the frequency of the clock signal result in changes in the time period required for one raster line of print data to be output by the printhead. Since the laser beam is scanned across the photosensitive surface at a constant rate of speed, a change in the time period required for one raster line of print data to be output by the printhead results in a change in the length of the raster line.
In embodiments of the invention, a method includes providing a plurality of clock signals. Each of the clock signals is associated with a respective one of a plurality of printheads in an electrophotographic machine. Each of the clock signals has a respective frequency. The method also includes using the printheads to create a plurality of raster lines. Each printhead is used to create the raster lines with a respective length. The length is dependent upon the frequency of the associated clock signal.
In other embodiments of the invention, an arrangement for use in an electrophotographic machine includes a clock signal source and a plurality of printheads. The clock signal source is configured to provide a plurality of clock signals, with each of the clock signals having a respective frequency. Each printhead is associated with a respective one of the clock signals. Each printhead is configured to print raster lines having lengths dependent upon the frequency of the associated clock signal.
In other embodiments of the invention, a method includes modulating a plurality of light beams in an electrophotographic machine. The modulating of each light beam is performed at a respective one of a plurality of modulating frequencies. The light beams are scanned across at least one photosensitive surface to thereby produce a plurality of raster lines. Each raster line has a respective length. The method also includes adjusting the length of at least one of the raster lines by changing the modulating frequency of at least one of the light beams such that at least two of the modulating frequencies are unequal.
As a result, the lengths of the raster lines produced by each of the printheads can be made to be equal. Thus, it is possible to register each color of toner relative to each of the other colors in the horizontal, scanning direction.
The above discussed features and advantages, as well as others, may be readily ascertained by those of ordinary skill in the art by reference to the following detailed description and accompanying drawings.